Mammography apparatus

ABSTRACT

A mammography apparatus capable of obtaining a clear breast image with reduced burden on the subject. The apparatus includes a radiation irradiation section and an object table connected to an arm such that they face each other, and the breast pressed on the object table using a compression paddle is imaged by rotating the arm according to the imaging direction. In the apparatus, the size of the breast is detected, and the relative position between the radiation irradiation section and object table is changed along the side of the object table facing the chest wall of the subject according to the detected size of the breast and the direction of rotation of the arm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mammography apparatus for obtaining abreast image.

2. Description of the Related Art

Breast cancer screenings are conducted using X-ray machines dedicated tobreast imaging (mammography apparatus), since the discovery rate ofearly stage cancer is increased by combining mammograms obtained by themammography apparatus with clinical breast examinations rather thandepending only on the clinical breast examinations.

General breast cancer screenings include a unidirectional imaging (twoimages) and a two-directional imaging (four images). In theunidirectional imaging, only the MLO (mediolateral oblique) imaging isconducted. In the MLO imaging, the breast is clamped and imaged from theoblique direction as shown in FIG. 12A. In order to enable the imagingfrom such direction, the mammography apparatus includes an arm to whicha radiation irradiation section and an object table are connected suchthat they face each other, and the object table is inclined by rotatingthe arm. In the two-directional imaging, both the MLO imaging and CC(craniocaudal) imaging are conducted. In the CC imaging, the breast isclamped and imaged from above as shown in FIG. 12B. The MLO imaging mayprovide an image having the widest area of a breast. The CC imaging isperformed to complement the MLO imaging, since it may visualize insideareas of a breast which are difficult for the MLO imaging to visualizeas an image. Generally, a unidirectional imaging and a two-directionalimaging is performed for each of the right and left breasts, and twoimages are obtained in the unidirectional imaging and four images areobtained in the two-directional imaging.

The breast includes mammary gland tissues, fat tissues and skin. Inorder to obtain a breast image required for giving an accurate.diagnosis, the breast needs to be pressed evenly as much as possible toimage small mammary glands and fat tissues on a film. Therefore, in themammography apparatus, the breast is imaged while it is clamped betweenthe object table and a compression paddle. If the pressing pressure istoo low, however, the mammary glands, fat, blood vessels and the likeoverlap with each other. Consequently, there may be a case in which atumor actually existing in the breast is not imaged. Thus, in order toobtain a good image, the breast needs to be pressed to a certain extentwhen imaged. That is, the pressing pressure needs to be controlled suchthe thickness of the breast becomes appropriate for imaging for eachtype of breast.

Further, AEC (automatic exposure control) for measuring the dose ofradiation transmitted through the breast is essential in order to stablyobtain an appropriate film density by the mammography apparatus. The AECis a sensor provided under the cassette, and in general, semiconductordetectors are disposed as the sensor. When imaging a breast of asubject, the position of the breast and the like are adjusted by theoperator so that the dose of radiation irradiated on the center of thebreast is measured by the sensor.

Therefore, when the two-directional imaging is performed for the rightand left breasts of a subject., such adjustments are required to be madefour times, causing a considerable burden on the subject.

Consequently, a mammography apparatus, in which an initially adjustedpressing pressure value or the like for each subject is stored forrepeatedly performed mammography screening, and imaging is performedusing the stored value from the next time, is proposed as described, forexample, in International Patent Publication No. WO01/054463.

Further, in order to obtain an image having an optimum density, it isnecessary to place the breast on the object table such that the centerof the breast corresponds to the center of the object table. The size ofthe breast, however, varies greatly between individuals, so that a largeobject table is used for imaging a large breast, and a small objecttable is used for imaging a small breast so that the breast is placed onthe center of the object table in the apparatus proposed in theaforementioned patent publication.

The burden on the subject may be alleviated by storing informationrelated to the pressing pressure and the like as proposed in theaforementioned patent publication, but it is difficult to invariablyplace the breast on the center of the object table. Consequently, theremay be a case that the obtained image is blurred or distorted.

Further, the size of the breast varies greatly between individuals, andif a small breast is imaged using a large object table, the position ofthe breast may be displaced from the center of the object table, so thatit is desirable to use object tables of different sizes according to thesize of breasts. But, the object table is expensive, so that it may bedifficult to invariably provide both the large and small platforms.

Thus, in order to obtain a clear breast image, it is desirable toperpendicularly irradiate radiation on the breast. When a small breastis imaged using a large object table, radiation may not always beperpendicularly irradiated on the breast. Consequently, if ananti-scatter grid is used, the effect of the grid is reduced since theradiation is incident on the breast at an angle, resulting in a blurredimage. Further, if the radiation is irradiated at an angle, themagnification rate differs depending on the position of the image, sothat the image is distorted.

It is an object of the present invention, therefore, to provide amammography apparatus capable of obtaining a clear breast image withreduced burden on the subject.

SUMMARY OF THE INVENTION

A first mammography apparatus of the present invention includes:

a radiation irradiation section for irradiating radiation on a breast ofa subject;

an object table having therein an image recording medium that receivesthe radiation to record image information thereon according to the doseof radiation transmitted through the breast, and a dose detector fordetecting the dose of radiation irradiated from the radiationirradiation section and transmitted through the breast;

an arm connecting the radiation irradiation section and object tablesuch that they face each other;

a compression paddle, disposed between the radiation irradiation sectionand object table, for pressing the breast on the object table;

an arm rotation means for rotating the arm according to the imagingdirection for the breast;

a breast size detection means for detecting the size of the breast ofthe subject; and

a relative position changing means for changing the relative positionbetween the radiation irradiation section and the object table along theside of the object table facing the chest wall of the subject accordingto the detected size of the breast and the direction of rotation of thearm when the arm is rotated by the arm rotation means.

The referent of “image recording medium” as used herein means a mediumon which an image of a subject through which radiation has transmittedis recorded. Specifically, such media include flat panel detectors,imaging plates, X-ray films, and the like. The flat panel detectorsinclude solid state detectors, TFTs (thin film transistors), and thelike.

The referent of “side of the object table facing the chest wall” as usedherein means the side of the object table that faces the chest wall whenimaging the breast, and the referent of “changing the relative positionalong the side of the object table facing the chest wall” as used hereinmeans that the relative position is moved substantially parallel to theside of the object table facing the chest wall.

The referent of “changing the relative position” as used herein meansthat the corresponding positions of the radiation irradiation sectionand the object table are changed by changing the position of either orboth of them.

The first mammography apparatus may further include a compression paddlesize detection means for detecting the size of the compression paddle.Here, the breast size detection means may be a means for detecting thesize of breast corresponding to the detected size of the compressionpaddle as the size of the breast of the subject.

Further, the breast size detection means may be a means for detectingthe size of the breast of the subject from the image informationrecorded on the image recording medium.

Still further, the relative position changing means may include a dosedetecting position changing means for changing the dose detectingposition of the dose detector along the side of the object table facingthe chest wall of the subject.

A second mammography apparatus of the present invention includes:

a radiation irradiation section for irradiating radiation on a breast ofa subject;

an object table having a breast placement surface;

a breast position detection means for detecting the position of thebreast on the object table along the side of the object table facing thechest wall of the subject; and

a relative position changing means for changing the relative positionbetween the radiation irradiation section and the object table along theside of the object table facing the chest wall of the subject such thatthe radiation irradiation section locates on a normal line to the breastplacement surface passing through the region of interest of the breastbased on the position of the breast detected by the breast positiondetection means.

The referent of “the region of interest” as used herein means the regionaround which radiation is irradiated when a breast image is recorded.When imaging a breast, it is desirable that the radiation is irradiatedcentered on the region of the breast where it is relatively thick. Forexample, the central region of the breast may be the region of interest.

The second mammography apparatus may further includes: an arm connectingthe radiation irradiation section and object table such that they faceeach other; an arm rotation means for rotating the arm according to theimaging direction for the breast; and a breast size detection means fordetecting the size of the breast on the object table. Here, the breastposition detection means may be a means for detecting the position ofthe breast on the object table from the angle of rotation of the armrotated by the arm rotation means and the size of the breast.

Further, the second mammography apparatus may further includes an objecttable height adjusting means for adjusting the height of the objecttable. Here, the breast position detection means may be a means fordetermining the position of the breast on the object table according tothe height of the object table adjusted by the object table heightadjusting means.

Still further, the second mammography apparatus may further includes: acompression paddle, disposed between the radiation irradiation sectionand object table, for pressing the breast on the object table; and acompression paddle inclination detection means for detecting aninclination angle of the compression paddle formed with respect to theobject table while the breast is clamped between the compression paddleand the object table and pressed. Here, the breast position detectionmeans may be a means for determining the position of the breast on theobject table according to the inclination angle of the compressionpaddle.

Further, the second mammography apparatus may further includes aradiation field aperture detection means for detecting the positionwhere a radiation field aperture for the radiation irradiation sectionis attached. Here, the breast position detection means may be a meansfor determining the position of the breast on the object table accordingto the position of the radiation field aperture.

According to the present invention, the relative position between theradiation irradiation section and object table is moved according to thesize of the breast such that the radiation is transmitted through thecentral region of the breast, so that an image without blur ordistortion may be obtained.

Further, the size of the breast may be readily detected by detecting thesize of the compression paddle.

Still Further, the center of the breast may be detected accurately bydetecting the size of the breast from the image information, and therebythe radiation may be irradiated on a more appropriate position of thebreast.

Further, the dose of radiation may be detected accurately by changingthe dose detecting position of the dose detector according to theposition of the breast.

Still further, the position of the breast on the object table may bedetermined without requiring any structural change in the mammographyapparatus by determining the position of the breast from the height ofthe object table.

Further, the position of the breast being pressed on the object tablemay be determined by determining the position of the breast from theinclination angle of the compression paddle.

Still further, the position of the breast on the object table may bereadily determined by determining the position of the breast accordingto the position where the radiation field aperture is attached with thecenter of imaging as the center of the breast.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the mammography apparatus according to afirst embodiment of the present invention.

FIG. 2 is a front view of the arm section of the mammography apparatus.

FIG. 3 is a drawing illustrating the rotation of the arm section of themammography apparatus.

FIG. 4 is a drawing illustrating the relationship among the compressionpaddle, solid state detector, and dose detector (example 1).

FIG. 5 is a schematic diagram of the object table of the mammographyapparatus, illustrating the inside thereof.

FIG. 6 is a schematic view of the solid state detector.

FIG. 7 is a drawing illustrating the solid state detector and theconnection of the current detection means to the detector.

FIG. 8 is a block diagram illustrating the detail of the currentdetection means and high voltage power source, and the connection of thedetection means and high voltage power source to the solid statedetector.

FIGS. 9A, 9B are drawings illustrating the arm section when the positionof the radiation irradiation section is moved to the center of a breast.

FIG. 10 is a drawing of the arm section illustrating the relationshipbetween the radiation irradiation section and an anti-scatter grid.

FIGS. 11A, 11B are a drawing of the arm section when the object table ismoved such that the center of the breast corresponds to the radiationirradiation section.

FIG. 12A is a drawing for explaining MLO imaging.

FIG. 12B is a drawing for explaining CC imaging.

FIG. 13 is a drawing illustrating that a CCD camera is installedadjacent to the radiation irradiation section.

FIG. 14 is a drawing illustrating that a strain gauge is provided in theobject table.

FIG. 15 is a drawing illustrating the relationship between the height ofthe object table and the position of a breast.

FIG. 16 is a drawing illustrating the relationship between theinclination of the compression paddle and the position of a breast.

FIG. 17 is a drawing illustrating the relationship between the radiationfield aperture and the position of a breast.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the accompanying drawings. FIG. 1 is aschematic view of the mammography apparatus according to a firstembodiment of the present invention, and FIGS. 2 and 3 are front viewsof the arm section of the mammography apparatus.

The mammography apparatus 1 includes: a radiation accommodation section3 having therein a radiation irradiation section 2; an object table 4having therein a recording medium holding section, such as a cassette orthe like, in which a recording medium, such as a flat panel detector 10or the like is accommodated; an arm 5 connecting the radiationaccommodation section 3 and object table 4 such that they face eachother. The arm 5 is attached to a base 6 through a spindle C.

The base 6 further includes: an operation section 8 for use by theoperator to control the height of the platform 4 (i.e., height of thearm 5), and inclination of the platform 4 (i.e., inclination of the arm5); and an arm moving means 9 for vertically and rotationally moving thearm 5 according to the input from the operation section 8.

As shown in FIG. 2, the radiation accommodation section 3 includes aradiation irradiation section moving means 86 for moving the radiationirradiation section 2. The radiation irradiation section moving means 86includes a pulse motor or the like, and moves the radiation irradiationsection 2 to the right or left parallel to the object table 4 along theside thereof facing the chest wall H of a subject.

The arm 5 includes an object table moving means 88 for moving the objecttable 4, which includes a pulse motor or the like, and moves the objecttable 4 to the right or left in the transverse direction of the arm 5along the side of the object table 4 facing the chest wall H.

The operation section 8 is constituted by an operation panel, a footpedal, and the like attached to the base 6 for use by the operator toinput instructions for changing the height and orientation of the arm 5to a position appropriate for imaging according to the physical size,breast size and position of a subject, and to input instructions forpressing the breast. For example, when a button on the operation panelis depressed once by the operator, information indicating the amount ofvertical or rotational movement determined by the depression of thebutton is sent to the arm moving means 9.

The arm moving means 9 includes: an arm rotation means 91 for rotatingthe arm 5; and an arm height adjusting means (object table heightadjusting means) 92 for adjusting the height of the object table 4 bymoving the arm 5 up and down.

The arm rotation means 91 rotates the spindle C attached to the base 6according to the inclination of the arm 5 inputted from the operationsection 8.

The object table adjusting means 92 moves the arm 5 up or down accordingto the height of the object table inputted from the operation section 8.

Between the radiation accommodation section 3 and object table 4, thearm 5 includes: amounting section 51 for mounting a compression paddle 7used for pressing a breast M on the imaging plate 4 from above; acompression paddle moving means 52 for moving the mounting section 51 upand down along the vertical direction of the arm 5; and a compressionpaddle size detection means 53 for detecting the size of the mountedcompression paddle 7. In the mean time, the compression paddle 7includes an insertion section 54, which is inserted into the mountingsection 51 of the arm 5.

A plurality of compression paddles 7 of different sizes is provided forbreasts of different sizes. The size of the large compression paddle 7is approximately 30 cm×24 cm, and the size of the small compressionpaddle 7 is approximately 24 cm×18 cm. Further, the length of theinsertion section 54 of the compression paddle 7 differs according tothe size of the compression paddle 7.

The compression paddle size detection means 53 is provided on themounting section 51 of the arm 5, which includes a dip switch or a photosensor that detects the length of the insertion section 54 inserted intothe mounting section 51 to detect the size of the compression paddle 7.Alternatively, it may be a barcode reader, in which case a barcodeindicating the size or model number of the compression paddle 7 isattached to the side thereof, and the barcode reader is disposed at aposition so that the barcode may be scanned by the barcode reader whenthe compression paddle 7 is inserted into the mounting section 51.

The object table 4 has a breast placement surface on which a breast M isplaced for imaging. Arranged inside of the object table 4 are: a flatpanel detector 10 that receives the radiation irradiated from theradiation irradiation section to record image information according tothe dose of radiation transmitted through the breast, and outputs imagedata representing the recorded image information; a dose detector 20 fordetecting the dose of radiation irradiated from the radiationirradiation section and transmitted through the breast, which isdisposed under the flat panel detector 10; and a dose detecting positionchanging means 21 for changing the position of the dose detector 20 tothe center of the breast, as shown in FIG. 4.

The arm 5 is attached to the base 6 through the spindle C, the rotationcenter of the arm 5, attached to the arm 5 at a position correspondingto the center of the flat panel detector 10 so that the rotation centerof the arm 5 corresponds to the center of the flat panel detector 10(FIG. 3).

Hereinafter, the structure of the object table 4 when a solid statedetector is used as the flat panel detector 10 will be described withreference to FIGS. 5 to 8.

As shown in FIG. 5, arranged inside of the object table 4 are: a readoutexposure light source section 73 used when reading out image informationrecorded on the solid state detector 10; a readout exposure light sourcesection moving means 75 for moving the readout exposure light sourcesection 73 in the sub-scanning directions; a current detection means 72for detecting currents flowing out of the solid state detector 10 whenscan exposed by the readout exposure light source section 73 to obtainimage signals; a high voltage power source section 71 for applying apredetermined voltage to the solid state detector 10; a pre-exposurelight source section 60 for irradiating pre-exposure light on the solidstate detector 10 prior to initiating imaging; a solid state detectormoving means 70 for moving the solid state detector 10 in the directionsapproaching the chest wall H of the subject and leaving therefrom(sub-scanning directions described above) within the object table 4; anda control means 80 for controlling the readout exposure light sourcesection 73, current detection means 72, high voltage power sourcesection 71, pre-exposure light source section 60, and moving means 70,75.

The solid state detector 10 is a direct conversion/optical readout typesolid state radiation detector, which receives recording lightrepresenting image information to record the image information as anelectrostatic latent image, and is scanned by readout light to generatecurrents according to the electrostatic latent image. More specifically,as shown in FIG. 6, it includes a glass substrate on which the followinglayers are arranged in the order listed below: a first conductive layer11 which is transparent to the radiation (recording light) transmittedthrough the breast M; a recording photoconductive layer 12 thatgenerates charges and shows electrical conductivity by receiving therecording light; a charge transport layer 13 that acts substantially asan insulator against charges having the polarity of latent image chargescharged on the first conductive layer 11, and substantially as aconductor for transport charges having the opposite polarity to that ofthe latent image charges; a readout photoconductive layer 14 thatgenerates charges and shows electrical conductivity by receiving readoutlight; and a second conductive layer 15 which is transparent to thereadout light. In addition, a storage section 17 is formed at theinterface between the recording photoconductive layer 12 and chargetransport layer 13.

The first conductive layer 11 and second conductive layer 15 areelectrode layers. The electrode of the first conductive layer 11 is aplate electrode which is two dimensionally flat, and the electrode ofthe second conductive layer 15 is a striped electrode constituted bymultitudes of elements (line electrodes) 15 a for detecting the recordedinformation as image signals arranged in stripes at a pixel pitch asshown in hatched lines in FIG. 5 (refer for example, to theelectrostatic recording medium described in Japanese Unexamined PatentPublication No. 2000-105297 for detail). The arrangement direction ofthe elements 15 a corresponds to the main scanning directions, and thelongitudinal direction of the elements 15 a corresponds to thesubs-scanning directions.

The solid state detector 10 has a long side of 30 cm and a short side of24 cm so as to be able to accept a large breast, and is arranged in theobject table 4 such that the long side directions correspond to the mainscanning directions and the short side directions correspond to thesub-scanning directions.

As for the readout exposure light source section 73, a light sourceconstituted by a line light source having a plurality of LED chipsarranged in a line, and an optical system for irradiating the lightoutputted from the line light source on the solid state detector 10 in aline. The entire surface of the solid state detector 10 is exposed byscanning the light source section 73 in the longitudinal direction ofthe striped electrodes of the detector 10, i.e., the sub-scanningdirections by the moving means 75 constituted by a linear motor with arequired distance between the light source section 73 and solid statedetector 10 maintained. The readout exposure light source section 73 andthe moving means 75 constitute the readout light scanning means.

FIG. 7 is a drawing illustrating the solid state detector 10 and theconnection of the current detection means 72 to the detector. As shownin the drawing, each element 15 a is connected to a charge amplifier IC33 through a printed pattern (not shown) formed on a TAB (tape automatedbonding) film 32 on the side of the detector 10 to be contacted with thechest wall H of a subject. Further, the charge amplifier IC 33 isconnected to a printed circuit board 31 through a printed pattern (notshown) formed on the TAB film 32. In the present embodiment, instead ofconnecting all of the elements 15 a to a single charge amplifier IC 33,several to several tens of charge amplifiers are provided, and everyseveral to several hundreds of elements 15 a are connected to eachcharge amplifier IC 33.

The embodiment of the current detection means 72 is not limited to thatdescribed above, and it may be embodied as so-called COG (chip on glass)in which the charge amplifier ICs are formed on the glass substrate 16instead of the TAB film 32.

FIG. 8 is a block diagram illustrating the detail of the currentdetection means 72 and high voltage power source section 71 provided inthe object table 4, and the connection thereof to the solid statedetector 10.

The high voltage power source section 71 is a circuit in which a highvoltage power source 711 and a bias switching means 712 are integrated.The high voltage power source 711 is connected to the electrostaticrecording section 10 through the bias switching means 712 that performsswitching for applying a bias voltage to the electrostatic recordingsection 10, or shunting the recording section 10 to ground. The circuitis a charge/discharge surge current suppression design, in which a peakcurrent that flows at the time of switching is limited to prevent thedestruction of the sections of the apparatus where the currents areconcentrated.

The charge amplifier IC 33 formed on the TAB film 32 includes: a numberof charge amplifiers 33 a, each connected to each element 15 a of thesolid state detector 10; a sample-and-hold (S/H) circuit 33 b connectedto each charge amplifier 33 a; and a multiplexer 33 c for multiplexingthe signal outputted from each S/H circuit. The current flowing out ofthe solid state detector 10 is converted by each charge amplifier 33 ato a voltage, which is sampled and held by the S/H circuit 33 b at apredetermined timing. The voltage sampled and held by each S/H circuit,which corresponds to each element 15 a, is sequentially outputted fromthe multiplexer 33 c so as to be switched in the arrangement order ofthe elements 15 a (corresponding to a part of the main scanning). Thesignals sequentially outputted from the multiplexer 33 c are inputted toa multiplexer 31 c provided on the printed circuit board 31, and thevoltage corresponding to each element 15 a is sequentially outputtedfrom the multiplexer 31 c so as to be switched in the arrangement orderof the elements 15 a, thereby the main scanning is completed. Thesignals sequentially outputted from the multiplexer 31 c are convertedto digital signals by an A/D converter 31 c, and stored in a memory 31b.

As for the pre-exposure light source section 60, a light source thatilluminates/extinguishes in a short time with very little afterglow isrequired. In the present embodiment, therefore, an external electrodetype rare gas fluorescent lamp is used. More specifically, as shown inFIG. 5, the pre-exposure light source section 60 includes: a pluralityof external electrode type rare gas fluorescent lamps 61 extending inthe direction perpendicular to the surface of the drawing; a wavelengthselection filter 62 provided between the florescent lamps and solidstate detector 10; and a reflector 63 for reflecting light emitted fromthe fluorescent lamps 61 to the solid state detector 10 effectively. Thepre-exposure light needs just to be irradiated on the entire surface ofthe second electrode layer 15, and a particular condenser means is notrequired, but a narrow luminance distribution is desirable. As for thelight source, for example, LED chips disposed two dimensionally may beused instead of the fluorescent lamps.

The moving means 70 is constituted by a linear motor or the like (notshown), and reciprocally translates the solid state detector 10 betweenthe imaging position and readout position.

In the present embodiment, an optical readout type solid state detectoris used as the flat panel detector as an example. Alternatively, a TFTreadout type solid state detector may also be used as the flat paneldetector as described, for example, in Japanese Unexamined PatentPublication Nos. 2004-80749, 2004-73256. In the TFT readout type solidstate detector, the signal charges stored in the storage section of thesolid state detection element are read out by scan driving the TFTsconnected to the storage section.

The dose detector 20 is disposed under the solid state detector 10, andas the dose detector 20, for example, an AEC sensor in whichsemiconductor detectors are disposed as the sensor for measuring thedose of radiation may be used. Alternatively, the dose detector 20 maybe adapted to detect the dose of radiation irradiated on the solid statedetector 10 (or TFT type flat panel detector). Hereinafter, in thepresent embodiment, the dose detector 20 will be described as the AECsensor.

The dose detecting position changing means 21 moves the dose detector 20parallel to the solid state detector 10 along the side of the objecttable 4 facing the chest wall H using a pulse motor or the like tochange the position for detecting the dose of radiation.

Hereinafter, the operation of the mammography apparatus 1 constructed inthe manner as described above will be described with reference to thecase in which a breast image of a subject is obtained by MLO imaging.

The breast is a solid organ having a certain thickness, so that if thebreast is imaged directly, a tumor may not be imaged due to interferenceby the mammary glands, fat, blood vessels, and the like. Therefore, inthe mammography screening, the breast is clamped by the compressionpaddle 7 to stretch it thinly so that the shadow of any small lump isclearly imaged with a small amount of radiation. In order to press thebreast evenly, it is necessary to uniformly apply a pressure thereto. Ifa large compression paddle 7 is used for a small breast, a gap isdeveloped around the breast, causing the compression paddle 7 to beinclined, and thereby the breast is not pressed evenly. Thus, it isnecessary to use a compression paddle 7 having an appropriate size forthe breast to be imaged.

Therefore, when imaging a breast of a subject, the operator selects anappropriate compression paddle 7 according to the size of the breast,and attaches to the mounting section 51 of the arm 5. When the selectedcompression paddle 7 is attached to the mounting section 51 of themammography apparatus 1, the size of the compression paddle 7 isdetected by the compression paddle size detection means 53.

When a subject stands on the side of the mammography apparatus 1, theoperator inputs the height of the object table 4 according to the heightof the subject, and the arm 5 is adjusted by the object table heightadjusting means 92 of the arm moving means 9 until the height of theobject table 4 becomes the inputted height. Further, the operator inputsthe angle of rotation of the arm 5 from the operation section 8according to the size and shape of the breast of the subject, and thearm 5 is rotated by the arm rotation means 91 until the angle ofrotation of the arm 5 becomes the inputted angle of rotation. Here,object table 4 is inclined at an angle in the range from 45 to 80degrees from the horizontal direction of the object table so that theobject table 4 becomes parallel to the chest muscles of the subject.Generally, the object table 4 is inclined around 60 degrees from thehorizontal direction for imaging. Depending on which of the right orleft breast is imaged, the direction of inclination of the object tableis different as shown in FIGS. 9A, 9B (FIG. 9A for right breast, FIG. 9Bfor left breast).

Imaging is performed with the upper side of the inclined object table 4contacted with the chest muscle. Therefore, if a small breast is placedon the breast placement surface of the object table 4 for large breast,and imaging is performed, the center of the breast does not correspondto the center of the object table 4, being always displaced to eitherside as shown in FIGS. 9A, 9B. Consequently, the radiation is irradiatedon the breast at an angle, rather than in perpendicular. This causes themagnification rate to differ between the central and edge regions of thebreast image, so that the image is distorted. Further, if ananti-scatter grid 24 is used for imaging, the orientation of the gaps ofthe grid 24 is caused not to correspond to the propagation direction ofthe radiation, as shown in FIG. 10, so that satisfactory effects are notobtained from of the anti-scatter grid 24. In order to obtain a clearand distortion-free breast image, the radiation needs to beperpendicularly irradiated on the center of the breast. Further, inorder to obtain a breast image having an appropriate density with asmall amount of radiation exposure, it is necessary to measure the doseof radiation transmitted through the central region of the breast by theAEC sensor 20.

Consequently, a relative position changing means moves radiationirradiation section 2 and object table 4 using the radiation irradiationsection moving means 86 and object table moving means 88 to change therelative position between the radiation irradiation section 2 and theobject table 4 such that the radiation irradiated from the radiationirradiation section 2 is incident perpendicularly on the center of thebreast M (i.e., such that the radiation irradiation section 2 locates ona normal line to the breast placement surface of the object table 4passing through the center of the breast).

First, the size of the breast is determined by the breast size detectionmeans based on the size of the compression paddle 7 detected by thecompression paddle size detection means 53. If the size of thecompression paddle 7 is small, the size of the breast M of the subjectis determined to be small. If the size of the compression paddle 7 islarge, the size of the breast M of the subject is determined to belarge.

When a small breast is imaged, the breast is recorded with its positiondisplaced to either side (FIGS. 9A, 9B). But, the direction of rotationof the arm 5 differs depending on which of the right or left breast M isimaged. Therefore, the displaced direction of the breast from the centerof the object table 4 may be known according to the direction ofrotation of the arm 5. Thus, the position of the breast M of the subjectplaced on the object table 4 is detected by a breast position detectionmeans based on the size of the breast detected by the breast sizedetection means, and the direction of rotation of the arm 5 rotated bythe arm rotation means 91. If the size of the breast M is large, thebreast is assumed to be placed adjacent to the center of the objecttable, and if the size of the breast M is small, the breast is assumedto be displaced from the center of the object table 4 always by adistance d.

According to the position of the breast M detected by the breastposition detection means, the radiation irradiation section 2 is movedby the radiation irradiation section moving means 86 by about thepredetermined distance d in the direction parallel to the object table 4along the side thereof facing the chest wall H, as shown in FIGS. 9A,9B. Further, the position of the AEC sensor is moved by the dosedetecting position changing means 21 by about the predetermined distanced along the side (long side) of the object table 4 facing the chest wallH, so that the AEC sensor 20 is positioned at a place corresponding tothe central region of the breast. The distance d is, for example, around3 cm, which corresponds to ½ of the difference in length between thelong side of the large compression paddle and that of the smallcompression paddle. The direction of the angle of rotation θ of the arm5 differs depending on which of the right or left breast is to beimaged, so that the moving direction of the AEC sensor is reversedaccording to the direction of rotation (or angle of rotation θ).

In the embodiment described above, the breast is moved upward and theimaging position is raised by the amount of h. Therefore, the arm 5 ismoved downward by the amount of h through the arm moving means 9, inorder to reduce the effort of the operator and the burden on thesubject.

The amount of h for the arm 5 to be moved downward may be obtained bythe following formula, when the travel distance of the AEC sensor is d,and the inclination of the arm 5 is θ.h=d×cos θIn the present embodiment, description has been made of a case in whichthe spindle C, the rotation center of the arm 5, is attached to the arm5 at a position corresponding to the center of the flat panel detector.But if the center positions thereof are displaced with each other, thetravel distance of the arm 5 in the upward or downward direction may becontrolled taking into account the displaced distance.

Alternatively, as shown in FIGS. 11A, 11B, the object table 4 may bemoved right or left across the arm 5 along the side of the object table4 facing the chest wall H by the object table moving means 88 withoutmoving the radiation irradiation section 2. Preferably, in this case,the AEC sensor 20 is moved in the direction opposite to the movingdirection of the object table 4 by the dose detecting position changingmeans 21, so that the relative position between the AEC sensor 20 andradiation irradiation section 2 is maintained, that is, the position ofthe AEC sensor 20 is maintained at a place corresponding to the centerof the breast. Here, a configuration may be adopted in which a pluralityof AEC sensors 20 is arranged and one of the sensors 20 whose variancein the position relative to the radiation irradiation section is minimalis used for detecting the dose of radiation, instead of changing theposition of the AEC sensor 20.

After the object table 4 is inclined at an optimum angle, the breast ispressed by the compression paddle 7.

Checking the pressurized state of the breast, the operator inputs aninstruction to gradually increase the pressure on the breast through theoperation panel, foot switch, or the like. Then, according to theinputted instruction, the compression paddle moving means 52 graduallypresses down the compression paddle 7 in the longitudinal direction ofthe arm 5. For example, the pressing pressure is increased by 1 kg everytime the foot switch is depressed, and the foot switch is depressedcontinuously until the thickness of the breast becomes appropriate forimaging. Alternatively, a configuration may be adopted in which thecompression paddle 7 gradually increases the pressure after it is moveddownward and touched on the breast.

After the pressing process is completed, radiation is irradiated fromthe radiation irradiation section 2 of the radiation accommodationsection 3, and the imaging is performed.

In the present embodiment, description has been made of a case in whicheither the radiation irradiation section or object table is moved, but aconfiguration may be adopted in which the both are moved to change therelative position thereof.

In the present embodiment, description has been made of a case in whichthe size of a breast is determined according to the size of thecompression paddle. But, the size of a breast may also be detected bypre-exposing the breast with a small amount of radiation by the breastsize detection means prior to imaging. More specifically, for example,the image obtained by the solid state detector 10 through pre-exposurehas different pixel values between the breast area and the area directlyexposed by the radiation, so that the breast area may be extractedthrough binarization of the image using a predetermined threshold value,and the size of the breast maybe determined based on the number ofpixels in the extracted breast area and the edge length thereof on theside of the chest wall H.

When the size of the breast is detected through the pre-exposure, thebreast size may be classified into a several levels according to thedetected size of the breast, and a plurality of distances for moving theradiation irradiation section 2 or object table 4 may be providedaccording to the level.

Alternatively, a CCD camera 25 may be installed adjacent to theradiation irradiation section 2 as the breast size detection means toimage the pressed breast, and the size of the breast area may bedetected from the image obtained by the CCD camera 25, as shown in FIG.13.

Further, a strain gauge may be provided in the object table 4, and thesize of the breast may be detected by calculating the weight of thebreast based on the reading of the strain gauge. Still further, the sizeof the breast may be detected from an image examined by using ultrasonicwaves immediately preceding the mammography imaging.

In the embodiment above, the description has been made of a case inwhich the position of the breast M is detected by the breast positiondetection means based on the size of the breast detected by the breastsize detection means and the direction of rotation of the arm 5 rotatedby the arm moving means. But, a configuration may be adopted in which astrain gauge 41 is provided on each side of the object table 4 as shownin FIG. 14, and the position of the breast M may be determined bychecking the balance between the outputs of the strain gauges. If apressure is applied on the object table 4, a strain is developed,thereby the resistance value of the strain gauge 41 is changed accordingto the strain. Thus, the position of the breast placed on the objecttable 4 may be determined from the difference in the output between thetwo strain gauges 41. For example, the position of a breast may beexpressed by the following Formula, using a difference in the outputbetween the two strain gauges 41 and a pressing pressure applied on thecompression paddle.Breast Position=fx(pressing pressure, difference in the output betweenthe two strain gauges)

Further, a configuration may be adopted in which a temperature sensor isprovided on the side of the object table 4 facing the chest wall H, andthe position of the breast is detected from the position where thesubject is contacting the object table 4.

When performing MLO imaging, the position of the breast M placed on theobject table 4 may be determined based on the height of the object table4 adjusted by the object table height adjusting means 92. Whenperforming MLO imaging, the object table 4 is inclined through rotatingthe arm 5 by the arm rotation means 91 for imaging. In addition, if thesubject is tall, the object table 4 is moved upward since the positionof the breast is relatively high. In this case, the distance from theunderarm to the center of the breast is also relatively large, so thatthe breast M may be placed at a position adjacent to the center of theinclined object table 4. But, if the subject is small, the breast M isnot placed at a position adjacent to the center of the object table 4but at a position slightly displaced upward from the center. Thus, atable like that shown in FIG. 15 may be provided for MLO imaging basedon the measuring results of the correspondence between the height of theobject table 4 and the position of the breast M of the subject obtainedin advance, and according to the table, the height of the object table 4and the approximate position of the breast M displaced from the centerof the object table 4 may be determined. Then, the radiation irradiationsection 2 or the object table 4 may be moved based on the value in thetable.

Further, if the breast M is large, imaging is performed with the objecttable inclined relatively small, but if the breast M is small, imagingis performed with the object table 4 inclined close to the vertical.Accordingly, the position of the breast M may also be determinedaccording to the inclination of the object table 4.

Still further, when the breast M placed on the object table 4 is pressedby the compression paddle 7, if the breast M is small, it is displacedto either side, so that the pressurized compression paddle 7 and theobject table 4 are not aligned in parallel, and the pressurizedcompression paddle 7 is inclined slightly with reference to the objecttable 4, as shown in FIG. 16. By detecting such inclination, theapproximate position of the breast M on the object table may bedetermined. Thus, a compression paddle inclination detection means 55for detecting an inclination angle δ of the compression paddle 7 withreference to the object table 4 may be provided on the insertion sectionof the compression paddle 7 or the like, and the position of the breastM on the object table may be determined by the breast position detectionmeans according to the inclination angle of the compression paddle 7.More specifically, the displacement of the breast M may be detected fromthe stress of the two insertion section 54 supporting the compressionpaddle 7.

If the compression paddle 7 is provided such that it is not inclinedwith reference to the object table 4, the position of the breast M maybe determined only from the distribution of the stresses incurred by thecompression paddle 7 or the object table 4.

Further, if imaging is performed by attaching a radiation field aperture18 to the radiation accommodation section 3 having therein the radiationirradiation section 2, as shown in FIG. 17, the radiation field aperture18 is attached such that the breast M is placed at the center of theimage. Thus, a radiation field aperture detection means 26 capable ofdetecting the position of the radiation field aperture 18 using a dialfor adjusting the radiation field size of the side of the object table 4facing the chest wall H or the side orthogonal to the side facing thechest wall H may be provided at the mounting section of the radiationfield aperture 18 of the radiation accommodation section 3, and theposition of the breast M on the object table may be determined accordingto the position of the radiation field aperture 18 attached to theradiation accommodating section 3.

The foregoing description has been made of a case in which the objecttable and the radiation irradiation section are moved relative to eachother so that the radiation irradiated from the radiation irradiationsection passes perpendicularly through the center of the breast. Theobject table and the radiation irradiation section may be moved relativeto each other so that the radiation is incident perpendicularly on theregion (region of interest) to optimize the recorded image.

As described in detail, by relatively moving the radiation irradiationsection and object table, the radiation may be irradiated on the centerof the breast, and thereby an image without blur or distortion may beobtained.

1. A mammography apparatus, comprising: a radiation irradiation sectionfor irradiating radiation on a breast of a subject; an object tablehaving therein an image recording medium that receives the radiation torecord image information thereon according to the dose of radiationtransmitted through the breast, and a dose detector for detecting thedose of radiation irradiated from the radiation irradiation section andtransmitted through the breast; an arm connecting the radiationirradiation section and object table such that they face each other; acompression paddle, disposed between the radiation irradiation sectionand object table, for pressing the breast on the object table; an armrotation means for rotating the arm according to the imaging directionfor the breast; a breast size detection means for detecting the size ofthe breast of the subject; and a relative position changing means forchanging the relative position between the radiation irradiation sectionand the object table along the side of the object table facing the chestwall of the subject according to the detected size of the breast and thedirection of rotation of the arm when the arm is rotated by the armrotation means.
 2. The mammography apparatus according to claim 1,wherein: the apparatus further comprises a compression paddle sizedetection means for detecting the size of the compression paddle; andthe breast size detection means detects the size of breast correspondingto the detected size of the compression paddle as the size of the breastof the subject.
 3. The mammography apparatus according to claim 1,wherein the breast size detection means detects the size of the breastof the subject from the image information recorded on the imagerecording medium.
 4. The mammography apparatus according to claim 1,wherein the relative position changing means includes a dose detectingposition changing means for changing the dose detecting position of thedose detector along the side of the object table facing the chest wallof the subject.
 5. The mammography apparatus according to claim 2,wherein the relative position changing means includes a dose detectingposition changing means for changing the dose detecting position of thedose detector along the side of the object table facing the chest wallof the subject.
 6. The mammography apparatus according to claim 3,wherein the relative position changing means includes a dose detectingposition changing means for changing the dose detecting position of thedose detector along the side of the object table facing the chest wallof the subject.
 7. A mammography apparatus, comprising: a radiationirradiation section for irradiating radiation on a breast of a subject;an object table having a breast placement surface; a breast positiondetection means for detecting the position of the breast on the objecttable along the side of the object table facing the chest wall of thesubject; and a relative position changing means for changing therelative position between the radiation irradiation section and theobject table along the side of the object table facing the chest wall ofthe subject such that the radiation irradiation section locates on anormal line to the breast placement surface passing through the regionof interest of the breast based on the position of the breast detectedby the breast position detection means.
 8. The mammography apparatusaccording to claim 7, wherein: the apparatus further comprises: an armconnecting the radiation irradiation section and object table such thatthey face each other; an arm rotation means for rotating the armaccording to the imaging direction for the breast; and a breast sizedetection means for detecting the size of the breast on the objecttable; and the breast position detection means detects the position ofthe breast on the object table from the angle of rotation of the armrotated by the arm rotation means and the size of the breast.
 9. Themammography apparatus according to claim 7, wherein: the apparatusfurther comprises an object table height adjusting means for adjustingthe height of the object table; and the breast position detection meansdetermines the position of the breast on the object table according tothe height of the object table adjusted by the object table heightadjusting means.
 10. The mammography apparatus according to claim 7,wherein: the apparatus further comprises: a compression paddle, disposedbetween the radiation irradiation section and object table, for pressingthe breast on the object table; and a compression paddle inclinationdetection means for detecting an inclination angle of the compressionpaddle formed with respect to the object table while the breast isclamped between the compression paddle and the object table and pressed;and the breast position detection means determines the position of thebreast on the object table according to the inclination angle of thecompression paddle.
 11. The mammography apparatus according to claim 7,wherein: the apparatus further comprises a radiation field aperturedetection means for detecting the position where a radiation fieldaperture for the radiation irradiation section is attached; and thebreast position detection means determines the position of the breast onthe object table according to the position of the radiation fieldaperture.